Don't forget it took 30-40 years for them to get that way. You might try some of that stuff that gets sprayed on tires to make them shiny. We all have various brands we prefer (convo for another day) or maybe even some vinyl dressing type stuff, used to keep dashboards and similar materials from drying out in the sun and keep them moist and clean.
Just an idea, but we have this stuff at work our detailers use for black plastic trim and tires and I spray it on the outside plastics, all the front, side markers, tail lights and roof lights, has a uv blocker in it to help slow down the effects of sun on the plastic. Been working good for me for a few years now since I started using it. Does great on old faded black plastic too, don't have any of that tho lol.